With the increasing prevalence of mammography screening programs, the majority of breast cancers are detected as small, non-palpable (or occult) lesions that are amenable to breast conserving treatment. Accurate localization of non-palpable breast cancers is key to allowing surgical removal of the complete tumor with adequate margins. If the tumor is not completely excised, patients need to undergo a further operation to remove any remaining cancerous tissue. Accurate localization also helps to avoid excision of excess breast tissue that could result in adverse cosmetic results. Accurate localization is required by other cancers such as colorectal, prostate and lung, as well as other conditions known by those of ordinary skill in this art.
The current gold standard for localization of non-palpable lesions during surgery is wire-guided localization (WGL). Although this technique is widely used, WGL has a number of disadvantages. First, it involves two separate procedures, and can present logistical and scheduling difficulties between radiology and surgery departments. Second, the positioning of the guidewire may not be optimal for achieving the desired cosmetic result in the subsequent surgery. Third, the hook wire can migrate away from the site of the lesion or become displaced during mammography or moving the patient. Fourth, the insertion of the wire can be painful for patients and finally, the risk of infection means that surgery usually needs to take place the same day as the wire insertion.
In order to overcome these disadvantages, other localization techniques have been developed. One such technique is Radioguided Occult Lesion Localization (ROLL) using a radiotracer injected into the tumor and detected by a handheld gamma probe. Although this removes the logistical complexity of WGL, the technique introduces the drawback of the use of radioactive materials, which require special handling and disposal procedures.
Magnetic markers are also used, and they overcome the inconvenience and logistical challenges that arise by using a radioactive material as a marker, and they also avoid the drawbacks of guide-wires. However, magnetic markers are relatively complex to manufacture compared with guide-wires.
All known marking devices, including wire guides and magnetic markers, are introduced through a hollow needle or cannula. To minimize patient discomfort, this needle is typically narrow in diameter. The small diameter of the needle constrains the marker cross section. For conventional biopsy needles this dimension is generally 14 to 18 gauge. This means that the needle has an internal diameter generally of 0.8 mm to 1.5 mm but may possibly be as large as 1.8 mm for certain needle designs. If a vacuum-assisted needle is used, the needle size is typically 11 gauge, with an internal diameter of 2.3 to 2.5 mm. Thus, the magnetic markers are typically constrained to be less than 1.5 mm in diameter. In practice, these size constraints limit the magnetic response and in turn the ease with which the marker can be localised with a magnetic probe. Therefore, a stronger magnetic response is desired.
Another challenge for magnetic biopsy markers is that to achieve an effective magnetic response, the volume of material needs to be maximised. This volume requirement results in a typically shaped marker having a length significantly greater than its diameter. Such markers are in the region of 1 mm to 12 mm, with a length to diameter ratio greater than 5. This aspect ratio results in a non-uniform magnetic response with a much stronger signal being obtained when the marker major axis is in line with a probe, and a weaker signal when the marker major axis is transverse to the probe. A more uniform response is generally desired.
Further, the marker is generally guided to its position and confirmed to be in place under ultrasound or stereotactic x-ray imaging. This means that it is desirable for the marker to be clearly visible under X-ray and ultrasound imaging, and preferably under MRI, which can also be used for this purpose.
What is needed is a marker that has a small amount of material without reducing the intensity of the detectable signal, and provides a more uniform response from any direction relative to the magnetic probe.
The present invention addresses this need.